Java Nio之基礎篇

Non-blocking IO

三大組件

• Channel
  ** FileChannel
  ** DatagramChannel
  ** SocketChannel
  ** ServerSocketChannel
• Buffer
  ** ByteBuffer
  ** CharBuffer
  ** DoubleBuffer
  ** FloatBuffer
  ** LongBuffer
  ** IntBuffer
  ** ShortBuffer
  ** MappedByteBuffer
• Selector
  ** 單線程處理多個Channel
  ** 向Selector中註冊Channel, 調用select()方法阻塞

基本用法

• channel是雙向的, 與buffer交互

public static void testFileBuffer() throws IOException {
    RandomAccessFile raFile = new RandomAccessFile("README.md", "rw");
    FileChannel inChannel = raFile.getChannel();

    ByteBuffer buffer = ByteBuffer.allocate(48);//capacity置爲48,即分配48字節的空間
    int byteRead = inChannel.read(buffer);//從channel中讀,向buffer中寫入數據
    while (byteRead != -1) {
        System.out.println(byteRead);
        buffer.flip();//轉換buffer到讀狀態,position重置爲0,limit置爲寫模式下的position
        System.out.println(new String(buffer.array(), "UTF-8"));
        while (buffer.hasRemaining()) {
            System.out.println((char)buffer.get());//從buffer中拿到數據,以字節爲單位
        }

        buffer.clear();//clear()清空buffer或compact()清除已讀數據
        byteRead = inChannel.read(buffer);
        System.out.println(byteRead);
    }
    raFile.close();
}

Buffer的三個屬性

• capacity
• position
• limit
• flip()方法
• rewind()方法
• clear()與compact()方法
• equals()與compareTo()方法

通道之間的數據傳輸

• 條件是其中一個通道是FileChannel
• FileChannel有兩個方法
  ** transferFrom()
  ** transferTo()
• 示例

public static void testChannelTransfer() throws IOException {
    RandomAccessFile fromFile = new RandomAccessFile("README.md", "rw");
    FileChannel fromChannel = fromFile.getChannel();
    RandomAccessFile toFile = new RandomAccessFile("nio-file.txt", "rw");
    FileChannel toChannel = toFile.getChannel();

    int position = 0;
    long count = fromChannel.size();
    toChannel.transferFrom(fromChannel, position, count);

    ByteBuffer buffer = ByteBuffer.allocate(1024);
    toChannel.read(buffer);
    System.out.println(new String(buffer.array()));
}

參考連接

• See ifeve-nio